It is a FXS to FXS adapter. It shouls sit betwen a normal phone line (line from the phone company) and your VOIP box.
To try it, install it betwen two normal phone lines to start with. Call the number of one of the phone lines. After it rings, the box should answer the call and present you the dial tone of the second line. Then you “overdial” with DTMF tones the second call.
If there is a period of silence of the box detects a busy tone , the call should be dropped.
If that works, you could disconnect the second phone line and replace it with your VOIP box (ATA 186, Ericsson adapter or 847v box)
Good luck.